Available Static Pressure vs. Flex Duct Equivalent Length
A residential contractor is checking a 3-ton air handler rated at 0.60 in. w.c. external static pressure at 1,200 cfm. The installed components are a filter rated 0.10 in. w.c., an evaporator coil rated 0.15 in. w.c., supply grilles totaling 0.03 in. w.c., and return grilles totaling 0.02 in. w.c. The longest supply path has a total effective length of 160 ft. The plan shows a 40-ft flexible branch to a 200-cfm bedroom, using 8-in. flex with two 90-degree bends, a takeoff, and a boot. The installer wants to use the remaining roll of 8-in. flex because it is already on the truck. The homeowner has complained that the bedroom is warm in cooling mode even though the rest of the house is comfortable.
1. Calculate the available static pressure for the duct system and the design friction rate. Show the arithmetic and explain why each pressure drop is included or excluded.
Read the worked answer
Available static = 0.60 - (0.10 filter + 0.15 coil + 0.03 supply grilles + 0.02 return grilles) = 0.30 in. w.c. The blower rating is external static at 1,200 cfm, so devices in the airstream that are not already included in that rating must be subtracted; otherwise they would be counted twice. Design friction rate = 0.30 / 160 ft = 0.001875 in. w.c. per ft, or about 0.188 in. w.c. per 100 ft. This is the average allowable loss per equivalent foot of the longest path, not a guarantee that every branch will perform acceptably.
2. Evaluate the proposed 8-in. flex branch using equivalent length. What is likely happening to bedroom airflow, and why?
Read the worked answer
Flex duct has a much higher equivalent length than its actual length because of the corrugations, sag, and fittings. A 40-ft run at a typical 2.5x to 3x multiplier is roughly 100 to 120 equivalent ft before adding two 90-degree bends, the takeoff, and the boot. That can consume nearly the entire 160-ft total effective length, leaving almost no allowance for the trunk and other fittings. At the design friction rate, 8-in. flex at 200 cfm is also at a relatively high velocity, so the branch likely cannot deliver 200 cfm. The bedroom is warm because it is receiving less air than design, not because the equipment is necessarily undersized.
3. Recommend a corrected branch design and describe how you would verify it.
Read the worked answer
Upsize the branch to 10-in. or 12-in. flex, shorten the flex run, use hard duct or metal elbows for the bends, and support the flex so it is fully extended without compression. Seal the takeoff and boot with approved mastic or tape, and avoid sharp bends at the boot. Recompute the equivalent length and friction rate for the revised path. Verify with a balancing hood that the bedroom receives approximately 200 cfm, check total external static pressure against the blower rating, and confirm the temperature split. If total airflow remains low, check the trunk and return path rather than continuing to upsize the branch.